Typically, in the intelligent transport system (ITS), a mobile communication device (a vehicle communication apparatus) performs communication with placed communication devices (roadside apparatuses) or with other mobile communication devices according as the situation demands. A method is being considered by which a mobile communication device receives a synchronization signal from a roadside apparatus upon entering the corresponding roadside apparatus area, and corrects its own synchronization signal.
However, in the past, the reliability of the synchronization signal transmitted by a placed communication device could not be determined by a mobile communication device.